Lori D. Daniels

Mountain Treelines: a Roadmap for Research Orientation

Abstract For over 100 years, mountain treelines have been the subject of varied research endeavors and remain a strong area of investigation. The purpose of this paper is to examine aspects of the epistemology of mountain treeline research—that is, to investigate how knowledge on treelines has been acquired and the changes in knowledge acquisition over time, through a review of fundamental questions and approaches. The questions treeline researchers have raised and continue to raise have undoubtedly directed the current state of knowledge. A continuing, fundamental emphasis has centered on seeking the general cause of mountain treelines, thus seeking an answer to the question, “What causes treeline?” with a primary emphasis on searching for ecophysiological mechanisms of low-temperature limitation for tree growth and regeneration. However, treeline research today also includes a rich literature that seeks local, landscape-scale causes of treelines and reasons why treelines vary so widely in three-dimensional patterns from one location to the next, and this approach and some of its consequences are elaborated here. In recent years, both lines of research have been motivated greatly by global climate change. Given the current state of knowledge, we propose that future research directions focused on a spatial approach should specifically address cross-scale hypotheses using statistics and simulations designed for nested hierarchies; these analyses will benefit from geographic extension of treeline research.

Direct and indirect impacts of climate change on forests: three case studies from British Columbia

Abstract Climate is an important driver of forest dynamics. In this paper, we present three case studies from the forests of British Columbia to illustrate the direct and indirect effects of climatic variation and global warming on forest composition and function. (1) Tree mortality rates in old forests of western North America have doubled in recent decades. Regional warming and water deficits directly affected tree death rates or indirectly increased insect and pathogen activity and wind storms causing tree deaths. Concurrently, tree density and basal area declined significantly, indicating lagged growth responses of surviving trees or long-term decline of these forests. (2) Yellow-cedar decline along coastal British Columbia and Alaska shows that small changes in average climatic conditions, coupled with extreme weather events, can have large ecological effects. A small persistent increase in mean temperatures has reduced snow-cover depth and duration. Coupled with extreme cold events which damage unprotected tree roots, these climatic changes are considered the primary cause of widespread yellow-cedar mortality. (3) Interactions between climate and disturbance are complex in the mountain forests of the East Kootenay region. Understanding historic climate–fire interactions is key to anticipating future frequent and severe fires. Here, climate change effects may be exacerbated by the cumulative effects of human land use, fire exclusion and mountain pine beetle outbreaks. We conclude that understanding past climate variation and its effects on forests help us to anticipate the potential effects of global warming.

Unstable climate−growth relations for white spruce in southwest Yukon, Canada

We used dendroclimatology to quantify inter-annual to multi-decadal climatic variation effects on white spruce radial growth in southwest Yukon, Canada. Local climate is dry and cold, such that tree growth was primarily moisture- rather than temperature-limited, although the mechanisms varied temporally. During the 20th century, significant increases in precipitation countered warming temperatures, so that heat−moisture indices have not changed significantly. Directional climatic change, superimposed on variation due to the Pacific Decadal Oscillation (PDO), resulted in unstable climate−growth relations. Prior to 1977, ring widths were positively correlated with previous growing season precipitation and warm temperatures had a negative impact, exacerbating moisture limitations in dry years especially during the cool, dry negative PDO phase (1946−1976). After 1977, correlations with previous growing season precipitation became negative and correlations with previous fall and winter precipitation and current year July and August temperatures became positive, although not statistically significant. These changes suggest precipitation and temperature increases over recent decades benefitted white spruce growth. Climate projections for this region include further temperature and precipitation increases, which may promote white spruce growth depending on the seasonality and interactions between temperature and precipitation. This study demonstrated the complexity of potential responses of white spruce to climate variation and change.

ASSESSMENT OF DENDROCHRONOLOGICAL YEAR-OF-DEATH ESTIMATES USING PERMANENT SAMPLE PLOT DATA

Abstract We combined crossdating with permanent sample plot (PSP) data to assess the precision and accuracy of year-of-death (YOD) estimates obtained by crossdating white spruce and lodgepole pine snags and logs. Crossdating indicated trees died between 1833 and 2006. Comparison of crossdated YOD dates for pairs of samples (n  =  121) showed that 90% of YOD estimates were within 10 years of each other. Of 100 trees that died after PSP establishment, 59 YOD dates were within the documented interval of death (IOD). Of the 41 inaccurate dates, 77% of YOD dates preceded the IOD midpoints and error increased with time since death. Regression models increased the accuracy of spruce YOD estimates for trees that had been dead ≥17 years, but the corrections were modest (e.g. +5 at 50 years). For pine, the correction models increased accuracy regardless of time since death and corrections were greater than those for spruce (e.g. +4 and +11 at 5 and 50 years, respectively). Precision and accuracy errors resulted from many factors including loss of bark, wood decay, lack of ring formation prior to tree death, and human error. Our results reinforce the need for multiple lines of evidence when reconstructing tree deaths using tree rings. We urge others with access to PSP data to assess the quality of crossdated YOD estimates. Ideally, PSP re-measurements intervals would be short and consistent, facilitating comparisons through time and among genera and locations.

Potential Impacts of Climate Change on Western Hemlock Looper Outbreaks

Abstract We calculated monthly soil moisture deficits from historical meteorological data from the south coast of mainland British Columbia to quantify warm and dry conditions at monthly and seasonal scales. Our main objective was to determine if climatic conditions antecedent to recorded outbreaks of western hemlock looper (Lambdina fiscellaria lugubrosa Hulst) may have triggered population growth. Periods of soil moisture deficits during the month of June were associated with the onset western hemlock looper outbreaks in the study area, as were warmer and drier conditions during the growing season two years prior to the first year of visible defoliation. Models of climate change predict that future summers in the Pacific Northwest will be warmer and potentially drier than present, and although other factors besides climate influence western hemlock looper population dynamics, the combined effects of more frequent droughts, changes in forest cover, and expanded range of western hemlock due to climate change may result in increased frequency, size and severity of outbreaks in the future.

Decay Dynamics of Coarsewood Habitat in Old-Growth Spruce and Pine Stands in the Rocky Mountain Foothills

Presentation at the 2009 NDP Information Session on abundance, distribution of decay classes and wildlife functions of spruce and pine snags and logs.

South American Dendroecological Fieldweek 2016: Exploring Dendrochronological Research in Northern Patagonia

Abstract The South American Dendroecological Fieldweek (SADEF) associated with the Third American Dendrochronology Conference was held in El Bolsón, Argentina, in March 2016. The main objective of the SADEF was to teach the basics of dendrochronology while applying specific knowledge to selected research questions. The course included participants and instructors from six different countries. This report describes activities of the course and briefly summarizes exploratory group projects. The Introductory Group developed an Austrocedrus chilensis chronology from 1629–2015 and documented a persistent decline in growth since 1977 which supports the fact that the current severe drought is the most severe in the 386-year record. Based on regional A. chilensis chronologies from 32° to 39°S Latitude, the Stream Flow Reconstruction Group developed a regional 525 year-long reconstruction from Río Chubut and found the most severe drought episodes from 1490 to the present occurred from 1680–1705, 1813–1828, 1900–1920, 1993–2002, and from 2011 to the present. The Drought Reconstruction Group used A. chilensis annual tree-ring width chronologies to develop preliminary spatial field reconstructions of the Palmer Drought Severity Index spanning the Central Andes region. The reconstructions explain up to 81% of the 1907–1975 PDSI variance, indicating this tree species is powerful for informing on historical drought especially in very arid domains. The Dendroecology Group documented three spreading fires since the 1850s with a 12-year return interval but lack of fire for the last 94 years; they also documented a persistent decline in their chronologies in recent years, dating back to 1965.

Tree-ring proxies of larch bud moth defoliation: latewood width and blue intensity are more precise than tree-ring width

Reconstructions of defoliation by larch bud moth (LBM, Zeiraphera diniana Gn.) based on European larch (Larix decidua Mill.) tree rings have unraveled outbreak patterns over exceptional temporal and spatial scales. In this study, we conducted tree-ring analyses on 105 increment cores of European larch from the Valais Alps, Switzerland. The well-documented history of LBM outbreaks in Valais provided a solid baseline for evaluating the LBM defoliation signal in multiple tree-ring parameters. First, we used tree-ring width measurements along with regional records of LBM outbreaks to reconstruct the occurrence of these events at two sites within the Swiss Alps. Second, we measured earlywood width, latewood width and blue intensity, and compared these parameters with tree-ring width to assess the capacity of each proxy to detect LBM defoliation. A total of six LBM outbreaks were reconstructed for the two sites between AD 1850 and 2000. Growth suppression induced by LBM was, on average, highest in latewood width (59%), followed by total ring width (54%), earlywood width (51%) and blue intensity (26%). We show that latewood width and blue intensity can improve the temporal accuracy of LBM outbreak reconstructions, as both proxies systematically detected LBM defoliation in the first year it occurred, as well as the differentiation between defoliation and non-defoliation years. This study introduces blue intensity as a promising new proxy of insect defoliation and encourages its use in conjunction with latewood width.

Deciphering the Complexity of Historical Fire Regimes: Diversity Among Forests of Western North America

Wildfire is a key disturbance agent in forests worldwide, but recent large and costly fires have raised urgent questions about how different current fire regimes are from those of the past. Dendroecological reconstructions of historical fire frequency, severity, spatial variability, and extent, corroborated by other lines of evidence, are essential in addressing these questions. Existing methods can infer the severity of individual fires and stand-level fire regimes. However, novel research designs combining evidence of stand-level fire severity with fire extent are now being used to reconstruct spatial variability in historical fire regimes and to quantify the relative abundance of fire severity classes across landscapes, thereby facilitating comparison with modern fire regimes. Here we review how these new approaches build on traditional analyses of fire scars and forest age structures by presenting four case studies from the western United States and Canada. Collectively they demonstrate the importance of ecosystem-specific research that can guide management aiming to safeguard human, cultural and biological values in fire-prone forests and enhance forest resilience to the cumulative effects of global environmental change. Dendroecological reconstructions, combined with multiple lines of corroborating evidence, are key for achieving this goal.

Dendroecology: Lessons Learned and Future Frontiers

Over the past four decades dendroecology has been instrumental in shaping contemporary understanding of how forests around the world change over time. Dendroecological research has provided important new insights into the functioning of temperate, boreal, and tropical forests at sub-annual, annual, decadal, and centennial time scales. Importantly, dendroecological research has provided the empirical framework for our current understanding of how individual trees respond to their environment, whether background climatic conditions, the effects of local competition, the impacts of biotic (e.g., herbivory by insects and vertebrates, disease) or abiotic (e.g., fire, wind, floods, drought) disturbances, or interactions among multiple environmental factors. The fundamental strength of dendroecology is in providing a universal framework for interpreting individualistic responses of trees to their environment.